How to determine the poly/vert counts of a fluid simulation?
When in object mode the info panel shows the vertex count of every object in the current scene which includes hidden objects, with a fluid simulation this changes each frame as the fluid mesh changes.

The frame before the fluid simulation starts will show the scene vertex count to use as a basis, lets start at frame -1 and say we have 16 vertices for a simple simulation with a cube for fluid and another for domain. Jump to frame 20 and we have 17184 vertices. By subtracting the 16 vertices we started with we have a fluid mesh with 17168 vertices. For an exact figure I believe the 8 vertices for the domain are included in the deformed fluid mesh so should not be subtracted from the vertex count of the frame.